Demographic research in India over the last two decades has focused extensively on fertility change and gender bias at the micro-level, and less has been done at the district level. Using data from the Census of India 1991–2011 and other sources, this paper shows the broad pattern of fertility transition and trends in the child sex ratio in India, and examines the determinants of the child sex ratio at the district level. During 1991–2011, while the Total Fertility Rate (TFR) declined by 1.2 children per woman, the child sex ratio fell by 30 points in the districts of India. However, the reduction in fertility was slower in the high-fertility compared with the low-fertility districts. The gender differential in under-five mortality increased in many districts of India over the study period. The decline in the child sex ratio was higher in the transitional compared with the low-fertility districts. The transitional districts are at higher risk of a low child sex ratio due to an increased gender differential in mortality and increase in the practice of sex-selective abortions. The sex ratio at birth and gender differential in mortality explains one-third of the variation, while region alone explains a quarter of the variation in the child sex ratio in the districts of India.